I think Mark's reccomendation that your turntable is being subject to vibrations from the loud audio. Possibly the low rumbling you are hearing is the feedback from the vibrations back into the phono cartridge? Get some long rca cables and move the turntable to another room. Play the source there, away from the loud audio and see if it is any better. If so then you can look into some sort of stabilizer for the turntable to rest on-

Good luck.

-Dave

M80s VP150 QS8s EP500s
ravenmanor.com/cinema/